Most signalling on telephone subscriber loops is performed between tip and ring, known as metallic signalling. The tip and ring have capacitance to the cable sheath and other tip-ring pairs. Occasionally, a loop may develop resistive faults to other pairs or to the sheath. Discussions of capacitance, impedance, or resistances from the tip and/or ring to other pairs and the sheath are often grouped, and referenced as longitudinal characteristics. They are also described as tip-ground and ring-ground components or measurements.
Subscriber loops and also analog inter-office trunks in telecommunications systems often run in near proximity to power lines and other sources of interference, and are susceptible to noise induction. These power lines and other sources induce current flow longitudinally in the pair. This longitudinally induced current flows in both tip and ring, in the same direction at the same time, and may result in several volts between the pair and ground. A percentage of this longitudinal voltage may appear differentially between the wires of the pair, and cause interference in the intended signals travelling on the pair. The amount of differential voltage (also called "metallic voltage" because it is between tip and ring) is determined by the relative impedances to ground of the individual wires in the pair. The relative impedances can be measured, and is called the longitudinal balance of the loop. Longitudinal balance is measured by inducing a specific signal into the loop longitudinally, and measuring the resulting metallic signal. The balance is basically the ratio of the stimulus over the measured signal. Higher values of balance indicate better matching of the tip-ground and ring-ground complex impedances, and infer less metallic noise on the loop. The balance of a loop must always be evaluated with the knowledge of the frequency of the measurement (stimulus frequency). The balance will vary with frequency, as the loop comprises a set of complex impedances which affect the sensitivity of the loop to longitudinal induction.
Balance measurements have traditionally been performed by disconnecting the pair from a telephone switch frame, and using a special purpose balance measurement instrument. As will be explained below, the instruments typically require significant expertise and manual adjustment before they present correct results. They are also difficult, if not impossible, to operate remotely. There are U.S. patents for automatic test sets for analyzing various parameters of a telephone subscriber's loop. For example, U.S. Pat. No. 5,073,919, issued Dec. 17, 1991 (Hagensick); U.S. Pat. No. 5,073,920, issued Dec. 17, 1992 (Masukawa et al); U.S. Pat. No. 4,459,436, issued Jul. 10, 1984 (Rubin); U.S. Pat. No. 4,446,341, issued May 1, 1984 (Rubin); U.S. Pat. No. 4,438,298, issued Mar. 20, 1984 (Rubin); and U.S. Pat. No. 4,467,147, issued Aug. 21, 1984 (Rubin), all teach mechanized testing equipment for measuring characteristics of subscriber's loops from the end at a central switching office. U.S. Pat. No. 4,459,436 above describes longitudinal balance measurement among other characteristics. In the patent, Rubin digitally generates stimulus signals and applies them to a loop under test. However, he uses a magnetic current sensor on the tip and ring. A special offset arrangement of the sensors detects a differential current between the pair and compensation factors are applied for calculation of the balance. The system still requires a great deal of analog operation and lacks accuracy.
The present invention allows a precise measurement of the balance remotely, automatically, relatively quickly, and without human intervention during the measurement. A foreign frequency at the stimulus frequency may cause significant error in the measurements. It is important to select a stimulus frequency which is not harmonically related to power line induction on the loops. The present invention allows selection of the stimulus frequency, so that the most appropriate frequency for the loop or trunk may be used. Typically, 200 Hz would be used on POTS (plain old telephone service) loops in North America, and 193 Hz would be suitable for POTS in other parts of the world which run 50 Hz primary power. Higher frequencies may be of interest for special purpose loops, trunks or services which are particularly susceptible to high frequency induction. The present invention rejects foreign frequencies which are at least 5 Hz away from the stimulus frequency.